1. Field of the Invention
This invention relates to a steering characteristic control apparatus for controlling the characteristic of a vehicle such as oversteer or understeer upon turning, and more particularly to a steering characteristic control apparatus for a vehicle which aims at a condition for ending control.
2. Description of Background
As one of techniques for controlling the behavior of a vehicle upon turning, a technique has been developed wherein braking force is applied to a particular wheel of a vehicle upon turning to control the steering characteristic of the vehicle to correct the posture of the vehicle upon turning in the turning direction to achieve stabilization in traveling of the vehicle and is disclosed, for example, in Japanese Patent No. 3, 257, 354 (hereinafter referred to as Patent Document 1).
According to the technique disclosed in Patent Document 1, when the oversteer of the vehicle upon turning is strong, the vehicle excessively turns to the inner side of turning and also the traveling path is displaced excessively to the inner side of turning, resulting in the possibility of spinning of the vehicle. Therefore, braking force is applied to turning outer wheels to suppress excessive turning round of the vehicle to the inner side of turning thereby to prevent the traveling path from being displaced to the inner side of turning (that is, to suppress the oversteer).
In this instance, if braking force is applied only to a front wheel from between the turning outer wheels, then the oversteer can be suppressed smoothly and efficiently without excessively slowing down the vehicle.
Further, when the oversteer is strong while a braking operation is performed during turning, either the braking force to the turning outer side front wheel is increased or the braking force to the turning outer side front wheel is increased and the braking force to the turning inner side rear wheel is decreased.
On the other hand, when the understeer of the vehicle upon turning is strong, the vehicle is less liable to be turned round to the inner side of turning and also the traveling path is liable to swerve to the outer side of turning, resulting in the possibility of a drift-out. Therefore, braking force is applied to turning inner wheels to cause the vehicle to turn round to the turning direction thereby to prevent the traveling path from being displaced to the outer side of turning (that is, to suppress the understeer).
In this instance, if braking force is applied only to a rear wheel from between the turning inner wheels, then the understeer can be suppressed smoothly and efficiently without excessively slowing down the vehicle.
Further, when the understeer is strong while a braking operation is performed during turning, either the braking force to the turning inner side rear wheel is increased or the braking force to the turning inner side rear wheel is increased and the braking force to the turning outer side front wheel is decreased.
Incidentally, according to the technique of Patent Document 1, while the behavior of a vehicle regarding the steering characteristic is controlled by applying braking force to the wheels, this is intended not only to achieve stabilization of the behavior of the vehicle regarding the steering characteristic by applying the braking force to the wheels but also to stabilize the behavior of the vehicle efficiency by applying braking force to a particular wheel (or increasing or decreasing the braking force) to generate a moment for stabilizing the behavior of the vehicle. In other words, according to the steering characteristic control, the behavior of the vehicle can be stabilized by applying a moment acting to cause the steering characteristic to approach the neutral steer.
In contrast, if it is intended to merely slow down the vehicle, also suppression of the output power for driving of the vehicle (that is, the engine torque) is applicable. For example, Japanese Patent Laid-Open No 2000-104582 (hereinafter referred to as Patent Document 2) discloses a technique wherein, if the behavior of a vehicle becomes unstable, then the fuel supply to the engine is intercepted to reduce the engine torque.
Where the behavior of a vehicle regarding the steering characteristic is controlled as described above, the control apparatus is usually configured such that it refers to a parameter value relating to the stability of the behavior of the vehicle and starts control when the parameter value representative of the behavior stability becomes lower than a control start reference value, whereafter the control is ended when the parameter value representative of the behavior stability of the vehicle exceeds a control end reference value.
The parameter value in this instance may be, for example, where it is used for the steering characteristic control, a yaw rate deviation calculated by subtracting a detected yaw rate (actual yaw rate) from a theoretical yaw rate (target yaw rate) determined from a traveling state and an operation state of the vehicle or may be a lateral acceleration acting on the vehicle.
As described above, in the behavior control by the steering characteristic control of a vehicle, the brake or the engine output power of the vehicle is controlled in order to secure the behavior stability of the vehicle. However, the brake or the engine output power of the vehicle should originally be controlled in accordance with the intention (operation) of the driver. Accordingly, the behavior control of the vehicle which is carried out independently of the intention of the driver should naturally be carried out only when it is required. In this regard, it is significant to set a control start condition and a control end condition such as a control start reference value and a control end reference value to appropriate values.
In the case of the control end condition, an end of control is determined from a state of the vehicle while the control is being carried out. However, even if the behavior of the vehicle is sufficiently stable while the control is being carried out, when the control is ended in this state, the behavior of the vehicle may sometimes be placed into an unstable state. In this instance, not only the behavior stability of the vehicle cannot be secured, but also control hunting takes place unfavorably. Accordingly, the control system is desirably configured such that, if the behavior stability of the vehicle cannot be secured when the behavior control of the vehicle is ended, then the control is continued, but if the behavior stability of the vehicle can be secured even if the behavior control of the vehicle is ended, then the control is ended rapidly.
Incidentally, the behavior stability of a vehicle is generally influenced by the state of the road surface of a road on which the vehicle travels, that is, by the coefficient of friction of the road surface. In particular, when the road surface is slippery (that is, when the road has a low μ), it is difficult to secure the behavior stability of the vehicle, but when the road surface is not slippery (that is, when the road has a high μ), it is easy to secure the behavior stability of the vehicle.
Also the behavior stability of the vehicle after the end of the control can usually be secured readily on a high μ road but can be secured less readily on a low μ road. Therefore, if the control end condition for a high μ road is set to a condition with which the behavior stability of the vehicle is comparatively low and the control end condition for a low μ road is set to another condition with which the behavior stability of the vehicle is comparatively high, then an end of the control can be determined further appropriately.
Meanwhile, turning of a vehicle which makes an object of behavior control of the vehicle can be classified roughly into unsteady or transient turning upon lane change or emergency avoiding steering which involves sudden changeover of the turning direction (such turning is hereinafter referred to also as lane change turning) and steady turning (hereinafter referred to also as simple turning) such as turning along a loop bridge or the like along which turning in the same direction continues and turning along a moderate S-shaped curve.
The simple turning is liable to be influenced by the road surface μ, and preferably the control end condition is configured such that, for a high μ road, it is set to such a low condition that the behavior stability is improved a little so that the behavior control may end at a point of time when the behavior of the vehicle is stabilized a little, but for a low μ load, it is set to such a high condition that the behavior stability is sufficiently high so that the behavior control may end at a point of time when the behavior of the vehicle is stabilized sufficiently.
On the other hand, upon lane change turning, changeover of the steering angle is performed suddenly, and also the turning round of the vehicle is likely to be performed suddenly. Therefore, a parameter value representative of the stability of the behavior of a vehicle is liable to be displaced to the unstable side. On the other hand, upon lane change turning, straightforward traveling is restored rapidly after a steering operation. Consequently, the behavior of the vehicle is liable to be stabilized after the control is ended. Therefore, if the control end condition for the lane change turning is set similarly to that for the simple turning, then a malfunction occurs.
In particular, if the control end condition when the vehicle runs on a high μ road upon simple turning (condition of such a degree that the behavior stability is improved only a little) is adopted as the control end condition for high μ road traveling, then the end of the control may be excessively earlier. In this instance, after the control ends once, the behavior stability of the vehicle drops, and the control start condition is established again and also the control itself is placed into instability.
On the other hand, if the control end condition upon low μ road traveling upon simple turning (a condition where the behavior stability is increased sufficiently) adopted as the control end condition upon low μ road traveling upon lane change turning, then the control continues for an unnecessarily long period of time, and this gives rise to such a malfunction that the vehicle is slowed down unnecessarily.